The present invention relates to an overcharge preventing device and an overdischarge preventing device for preventing overcharge and overdischarge of a nonaqueous electrolyte secondary battery such as lithium ion secondary battery.
Construction of a nonaqueous electrolyte secondary battery, e.g., a lithium ion secondary battery is shown in FIG. 7. Referring to FIG. 7, reference numeral 1 generally denotes a lithium ion secondary battery employing LiCoO.sub.2 as an active material 4 of a positive electrode 2 and carbon having a graphite structure as an active material 6 of a negative electrode 3. The active material 4 of the positive electrode 2 is held by a current collector 5 made of Al, and the active material 6 of the negative electrode 3 is held by a current collector 7 made of Cu. The active materials 4 and 6 are opposed to each other with a separator 8 interposed therebetween. An organic electrolyte 9 is filled between the active materials 4 and 6. A voltage reaction of the lithium ion secondary battery 1 is expressed by the following formula 1. ##STR1##
As shown in FIGS. 8 and 9, the charging and discharging characteristics of the battery 1 has a remarkable correlation between a battery capacity, that is, a charging and discharging energy and a terminal voltage of the battery (what is called a battery voltage). A solid line I in FIG. 8 is a graph illustrating the relation between the battery voltage and the charging energy. It is apparent from FIG. 8 that the battery voltage increases with an increase in the charging energy.
A solid line II in FIG. 9 is a graph illustrating the relation between the battery voltage and the discharging energy. It is apparent from FIG. 9 that the battery voltage decreases with an increase in the discharging energy.
The battery 1 has a design voltage a (see FIG. 8) to be defined by a battery component material and a battery design. To charge the battery 1 beyond the design voltage a is called overcharge. The overcharge causes the problems of (1) deposition of Li metal on the negative electrode 3, (2) decomposition of the active material 4 of the positive electrode 2 and deposition of Co metal or Co compounds on the negative electrode 3 due to cobalt ion generated by the decomposition of the active material 4, and (3) decomposition of the organic electrolyte 9. The decomposition of the Li metal, the Co metal and the Co compounds causes short-circuit of the positive and negative electrodes, and the decomposition of the active material 4 of the positive electrode 2 and the organic electrolyte 9 causes a remarkable deterioration of the battery 1. Accordingly, if the overcharge is not substantially avoided, the reliability of the battery cannot be ensured.
In discharging the charged battery 1 by connecting the same to an external load, the battery voltage decreases as shown in FIG. 9. When the discharging is continued, the battery voltage reaches a dissolution voltage b of the current collector (Cu) 7 of the negative electrode 3. To discharge the battery 1 after the dissolution voltage b is reached is called overdischarge. The overdischarge naturally causes ionization of the copper (Cu) of the current collector 7, and the copper ion is eluted into the electrolyte 9. The elution of the metal of the current collector 7 causes deterioration of a current collecting function and falling of the active material 6 of the negative electrode 3, reducing a capacity of the battery 1. Furthermore, the copper ion thus eluted is abnormally deposited on the negative electrode 3 in carrying out the charging in the next time, causing short-circuit of the positive and negative electrodes. Accordingly, the overdischarge must also be avoided.
Conventionally, the following techniques have been proposed to cope with the overcharge and the overdischarge. As the measures to the overcharge, there have been proposed (1) control of a charging voltage by a charger and (2) a current cutting device utilizing an internal pressure of the battery.
The above measure (1) to the overcharge by the charger is to control a charging terminal voltage of the battery in charging. This measure is sufficiently effective in case of a unit battery or parallel connected unit batteries. However, batteries are used often in series connection. In this case, a terminal voltage of the series connected batteries can be controlled, but the battery voltage of each unit battery cannot be controlled. Accordingly, when at least one of the unit batteries connected in series is short-circuited, the other unit batteries become overcharged in charging. Thus, this method is not a perfect measure to the overcharge.
As the current cutting device mentioned as the above measure (2), it is known to utilize an increase in internal pressure of the battery in overcharging, thereby mechanically cutting a current lead wire to cut off a charging current. This method is intended not to prevent the overcharge itself but to prevent breakdown of the battery due to an abnormal increase in temperature and internal pressure of the battery after the progress of the overcharge. Moreover, once the charging current is cut off, the battery cannot be used any more.
As the measure to the overdischarge, it is known to select the metal of the current collector of the negative electrode so that the dissolution voltage may be close to zero as the battery voltage. For example, when Cu is replaced by Ni, the effect is observed, but it is not perfect. In case of series connected batteries, the overdischarge of one of the batteries necessarily proceeds because of a difference among the individual unit batteries, causing a remarkable reduction in charging and discharging cycle life.